With the increase of video services in networks, the capacity of the entire network is required to be higher. At present, a rate upgrade may be needed for an optical network, and a next generation optical network has a rate of 40 Gb/s or higher. For example, a rate of an Ethernet is upgraded by 10 times. The 100 GE is being discussed currently, and the 100 Gb/s optical transmission technology has become a hot topic nowadays.
For a 40 Gb/s optical transmission system, multiple optical modulation code patterns exist, for example, a non-return-to-zero (NRZ) code, a return-to-zero (RZ) code, a carrier suppressed return-to-zero (CSRZ) code, an optical duobinary (ODB) code, and a DQPSK code. For a 100 Gb/s optical transmission system, optical modulation code patterns such as DQPSK, vestigial side band (VSB), NRZ, and ODB exist. Among them, the DQPSK has become a mainstream modulation code pattern of 40 Gb/s and 100 Gb/s. With the DQPSK modulation code pattern, a baud rate of the system can be reduced, thereby lowering the requirements for underlying optical network facilities.
In the prior art, two main solutions for generating a DQPSK optical signal exist, which are described as follows.
One solution is a parallel solution, which is currently the most commonly used solution. A schematic structural view of the parallel solution is shown in FIG. 1. An optical signal output from a laser is split into two optical signals having the same light intensity through a Y-branch beam splitter. One optical signal is modulated and then phase-shifted to obtain a phase-shifted optical signal. The other optical signal is modulated. The phase-shifted optical signal and the modulated optical signal are combined through the Y-branch beam splitter to obtain a DQPSK optical signal.
The solution has the following disadvantages. First, the Y-branch beam splitter has a significant influence on the entire apparatus. The realization for the Y-branch beam splitter to split one optical signal into two optical signals having the same light intensity may be complex and expensive. Second, it is difficult for the solution to control a light intensity percentage ratio of an X-polarized component to a Y-polarized component of the two beams of light, which causes unstable light intensity so as to deteriorate the transmission performance. Moreover, the change of external factors (for example, temperature and shock) may result in the change of the percentage ratio of the polarized components.
The other solution is a serial solution. The solution employs a Mach-Zender modulator (MZM) to obtain a binary phase shift keying (BPSK) optical signal. The BPSK optical signal passes through a phase modulator to obtain a DQPSK optical signal.
The solution has the disadvantage that the phase modulator requires a much high high-frequency response. For example, in a 40 G GB/s DQPSK system, the phase modulator is required to ensure a phase modulation of 90 degrees at a baud rate of 20 G and in a 100 G GB/s DQPSK system, the phase modulator is also required to ensure a phase modulation of 90 degrees at a baud rate of 50 G, which are difficult to realize.